I use dual HD 6970s right now, but have used a assorted amounts due to well being hooked and having the upgrade itch. I game at 6000 x 1080, which is essentiall 1920x3 by 1080, I just have bezel correction which makes my playable resolution that.

I haven't had the chance to play BF3 yet as I did not have early access, was busy all of yesterday and the 2 mins I had to play it, I was stuck in windowed mode which meant no crossfire enabled and horrible frames.

This is a misconception, you do not add up the amount of vRam (video ram) when you have multiple cards. Each card has whatever amount it comes with, it just means you have two cards processing images independently with the amount of onboard memory for it. With lower end cards, such as with just regular sli or crossfire, some people have gotten micro stutter due to the vRam not been large enough and there being a delay between the two cards to update the screen. This doesn't really happen (noticeable to me) with the high end or flagship cards.